Disc drives are used for storing digital information on rotating disc or discs. In the disc drive, the discs rotate at high speed supported on the hub of a spindle motor, and a transducer flies over the surface of the disc. The transducer records and/or reads information on the disc surface. The transducer is moved radially across the surface of the disc so that the different data tracks can be read back.
Over the years, as storage density has increased and the size of the storage system has decreased, the trend has led to greater precision and lower tolerance in the manufacture and operation of disc drives. For example, to achieve increased storage densities, the transducer must be placed increasing close to the surface of the disc, and the disc must rotate in a single plane without wobble or runout in the disc rotation.
From the foregoing, it is apparent that the bearing assembly which supports the storage disc on the spindle motor is of critical importance. A typical bearing assembly comprise ball bearings supported between a pair of races which allow a hub of the storage disc to rotate relative to a fixed member. However, such ball bearing assemblies have mechanical problems such as wear, runout, and manufacturing difficulties. Therefore, an alternative design which is now being widely adopted is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with the ball bearing assembly which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or runout between the rotating and fixed members. Moreover, the use of fluid in the interface area imparts damping effects to the bearing which helps reduce nonrepeatable runout.
However, the same characteristics require that the gap between the rotating and the stationary surfaces be extremely small, measured in microns. The fact that these two relatively rotating surfaces are very close together, and highly lubricated, creates a stiction problem when rotation stops. That is, at startup the hydrodynamic bearing (which is also sometimes referred to as a herringbone bearing because of the patterns which are impressed on one of the relatively rotating surfaces) has the stationary and rotary surfaces which are in intimate contact. The large contact area requires a relatively large torque to overcome both the stiction forces and the frictional forces. Hydrodynamic bearing designers have been dealing with this problem by reducing the area of the herringbone, which in turn reduces the stiffness of the bearing. None of the methods used to date address the fundamental problem of decoupling the problem of stiffness from the stiction problem in the bearing land (ungrooved surface) area.
During startup of hydrodynamic bearings (also known as herringbone bearings, stepped bearings, Rayleigh bearings), the stationary and rotary surfaces are in intimate contact producing high localized stress areas due to imperfections in flatness and machining burrs and defects. As long as the Herringbone or Rayleigh steps surfaces are in intimate contact the tribological performance of the interface is going to be unreliable and very difficult to reliably produce. The field of hydrodynamic bearing design has been attempting for many years to resolve these tribological problems without success.
The present invention is intended to provide a solution to these and other problems and offers other advantages over the prior art.